1. Field of the Invention
The present invention relates to a boiling water reactor core including a water rod, a boiling water reactor including the core, and a method of operating the boiling water reactor.
2. Description of the Related Art
Usually, a core of a boiling water reactor (BWR) includes a number of fuel assemblies arrayed in the form of a square lattice. Each of the fuel assemblies comprises a number of fuel rods arrayed in the form of a square lattice, and at least one water rod disposed near a center position of the array of the fuel rods. A channel box surrounds an outer periphery of a fuel bundle of the fuel rods held together by a fuel spacer. The fuel rods each include a number of fuel pellets filled in a cladding pipe made of an zirconium alloy, for example. Most of the fuel pellets are each made up of enriched uranium having an increased concentration of U-235 that is a fissile material.
The water rod is a hollow pipe which takes in water as a coolant through an opening formed in its lower portion, and allows the water to flow out through an opening formed in its upper portion. This water circulation increases a neutron moderating effect in a central portion of the fuel assembly in horizontal section, and enhance nuclear fission in the central portion. Various structures of the water rod have been proposed in the past. Recently, a water rod including a rising pipe and a falling pipe communicated with the rising pipe has been proposed. Such water rods are disclosed in JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992), pp. 55-60 (especially FIG. 5 in page 58). The rising pipe has a lower end opened below an upper surface of a lower tie plate which holds lower end portions of fuel rods. The falling pipe has a lower end opened above the upper surface of the lower tie plate. In the disclosed water rods, a difference between a pressure at the lower end opening of the rising pipe and a pressure at the lower end opening of the falling pipe is equal to a pressure loss occurred in a flow passage above the upper surface of the lower tie plate. During reactor operation, therefore, the surface of water in the rising pipe takes a level that gives a density head corresponding to the pressure loss, and the level of the water surface changes vertically depending on an increase and decrease in flow rate of cooling water passing through a core (i.e., in core flow rate). The above-mentioned water rods are called spectral shift rods.
A nuclear reactor is shut down to replace a part of fuel assemblies in a reactor core. During shutdown of the reactor core, the fuel assemblies whose lifetime has expired are taken out of the core, and new fuel assemblies are loaded in the core. A period of reactor operation from start-up of the reactor after loading of the new fuel assemblies to next shutdown of the reactor for replacement of the fuel assemblies is called a operation cycle.
Because of the loading of the new fuel assemblies, excess reactivity is increased at the beginning of the operation cycle. Control of the excess reactivity is important to keep constant a reactor power at the rated power (100% power) in the operation cycle. The excess reactivity is controlled by a burnable poison filled in a part of fuel rods and by insertion of control rods into the core. In the case of using a spectral shift rod, however, the excess reactivity is controlled by adjusting the level of the water surface in the rising pipe instead of manipulating the control rods. At the start-up of the reactor, the control rods are all withdrawn out of the core.
During the rated power operation at the early period of the operation cycle, the core flow rate is relatively small and the water surface is formed in the rising pipe. In this condition, the void fraction increases in upper portions of the fuel assemblies, and nuclear fission is suppressed to hold down the excess reactivity. Approaching the end of the operation cycle, the core flow rate increases and the level of the water surface in the rising pipe rises. This phenomenon is equivalent to a lowering of the void fraction. Nuclear fission is more activated with a rise of the level of the water surface. At the time of reaching a certain point in the operation cycle, the rising pipe is filled with water.
The above-cited JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992) describe adjustment of the core output during the rated power operation. At the beginning of the operation cycle, the neutron moderating effect is reduced and U-235 is less consumed. At the same time, U-238 occupying a large part of the fuel material absorbs fast neutrons, whereupon Pu-239 is produced. Filling the rising pipe with the water in the late period of the operation cycle to enhance the neutron moderating effect promotes nuclear fission of U-235 and Pu-239. Consequently, production of Pu-239 is promoted and nuclear fission of Pu-239 is developed in the late period of the operation cycle, thus resulting in a saving of U-235 (uranium conservation).
In each operation cycle of a BWR, the core flow rate and the control rod manipulation are controlled in an automatic manner during a period from an low end at the automatic flow control range (described later), which represents a setting flow rate with respect to the core flow rate, until the reactor core power reaches the rated power. When the core flow rate is smaller than the low end at the automatic flow control range, the core flow rate and the control rod manipulation are controlled in a manual manner. In a predetermined range of the core flow rate from the rated power operation to the low power operation (hereinafter referred to the non-rated power operation), the core flow rate can be automatically controlled. At a low end of the automatic flow control range corresponding to the non-rated power operation (i.e., at the low end at the automatic flow control range), it is required to satisfy certain minimum restrictions of the core preset on the nuclear thermal-hydraulic stability from the standpoint of ensuring safety.
The above-cited JP, A, 63-73187, U.S. Pat. No. 5,023,047, U.S. Pat. No. 5,640,435, and Hitachi Hyoron, Vol. 74, No. 10 (1992) primarily intend to adjust the power during the rated power operation, namely to vertically change the level of the water surface in the rising pipe during the rated power operation. As an incidental advantage, the spectral shift rods described in those known references have a possibility that the nuclear thermal-hydraulic stability at the low end at the automatic flow control range during the non-rated power operation is slightly improved. Those spectral shift rods have however drawbacks below.
Because of a water surface being present in the rising pipe during the rated power operation, supposing if the core flow rate should abruptly increase due to, e.g., an abnormal condition occurred in a pump control system, this would possibly cause an abrupt rise of the water surface and abruptly increase an amount of water in the reactor core, thus resulting in a difficulty in suppressing a rising rate of the reactor power. In other words, there has been such a risk that influences of a transient event such as an increase of the core flow rate cannot be always avoided with sufficient reliability.
Additionally, U.S. Pat. No. 4,708,846 discloses, though not a spectral shift rod, a water rod having a rising pipe and a falling pipe which are communicated with each other. The disclosed water rod primarily intends to improve an ability of cooling fuel rods with water flowing out of an outlet of the falling pipe. The height from a lower end of the fuel effective length to the outlet of the falling pipe is set to be not less than 65% but not more than 75% of the fuel effective length.